Internal combustion process and engine

ABSTRACT

An internal combustion process and engine in which the chemical energy of a fuel is converted into heat by timed combustion using self-ignition and into kinetic energy by expansion of the hot combustion gases and is transmitted by way of a piston to a shaft. The combustion-supporting air is compressed substantially isothermally, then heated by the hot exhaust gases. Some of the compressed hot air is diverted to preheat and carburet and/or compress a fuel and the remainder of the combustion-supporting air and the combustible gas evolved are supplied separately and in substantially stoichiometric quantities to a burner nozzle, mixed therein and burned by self-ignition with a reduced excess of air and without pressure increase. The energy of the combustion gases which expand without cooling is transmitted to a piston and the exhaust gases are removed from the cylinder chamber.

The invention relates to an internal combustion process in which thechemical energy of a fuel is converted into heat by timed combustionusing self-ignition and into kinetic energy by expansion of the hotcombustion gases; the invention also relates to a reciprocating internalcombustion engine operating in accordance with the process.

In conventional thermo dynamic circuit processes such as occur e.g. insteam engines and reciprocating engines, maximum efficiency can be anywhere between 10 and a maximum of 40%. The main reasons for such lowefficiencies are the considerable heat losses associated with expansionof the highly compressed gaseous media and with compression of thecombustion-supporting air in the cylinder (more particularly in the caseof internal combustion engines). Because of the properties of thematerials used for pistons and cylinders and because of the propertiesof normal lubricating oils, the wall temperature of the cylinders orthose rubbing surfaces of the liners which have to be lubricated mustnot exceed temperatures of from 220°-250° C. Because of the hightemperatures of combustion, the average temperatures in the cylinderchamber are considerably more than 2,000° C., and so much of the heatwhich the hot exhaust gases transfer to the cylinder walls is removed bywater or air cooling.

This unavoidable cooling of the cylinder walls is the reason for a largeproportion of the in all considerable heat losses. Because of theconsiderable temperature differences between the interior of thecylinder and the cooled cylinder walls, considerable amounts of heat andenergy are yielded to the cylinder walls and removed by coolants, moreparticularly during combustion of the mixture, expansion of the hotexhaust gases and during the exhaust stroke. In Otto and diesel enginesthe amount of heat thus lost may be as much as 30% of the heat energy ofthe fuel supplied.

Another disadvantage of cooling the walls of the cylinder chamber orcombustion chamber is that combustion near the cooled walls proceeds ata lower temperature than at the center, with the result of incompletecombustion, a further power loss and the presence of a high proportionof unburned ingredients in the exhaust gases.

Yet another factor adversely affecting the energy balance sheet in suchengines is that combustion and expansion occur in the same chamber. Thehot walls and possibly parts of the residual hot exhaust gases yieldheat to the media which it is required to compress and thus furtherincrease the temperature of the gases during compression beyond theincrease arising from such compression; consequently, more mechanicalwork is needed to achieve a required final pressure than would be needede.g. if compression were to be substantially isothermal. Unfortunately,the only way of removing most of the heat produced by compression wouldbe very intense cooling of the cylinder walls, with the result that theheat losses in expansion would increase and combustion would beimpaired.

Another factor responsible for losses in Otto and diesel systems is thedead space between the cylinder head and the piston crown in the topdead center position, such space serving to receive the compressed andpossibly partly burned combustible gases and appearing on the energybalance sheet as a distance/power loss.

Since the duration of combustion per piston stroke also depends upon thespeed of engine rotation the time available for combustion is very shortand, more particularly in the case of high-speed engines, ofteninadequate; to compensate for this disadvantage, recourse is had toincreasing the excess of air in diesel engines and to adjusting orincreasing preignition, because of inadequate external ignition, in Ottoengines; unfortunately, such action decreases efficiency in both cases.

It is an object of the invention to provide a thermo dynamic circuitprocess having greatly reduced heat losses and to provide acorrespondingly operating reciprocating internal combustion engine ofcorrespondingly higher thermal efficiency and therefore having greatereffective efficiency.

According to the invention, therefore, the combustion-supporting air iscompressed substantially isothermally, then heated by the hot exhaustgases; some of the compressed hot air is diverted to gasify, preheatand/or compress a fuel; the combustion-supporting air and thecombustible gas evolved are supplied separately and in substantiallystoichiometric quantities to a burner nozzle, mixed therein and burnedby self-ignition with a reduced excess of air and without pressureincrease; and the energy of the combustion gases which expand withoutcooling is in known manner transmitted to a piston.

According to an advantageous development of the process, to prevent overheating in the combustion chamber super heated steam can be added to thecombustion-supporting air or to the air-gas mixture. As a furtheradvantageous improvement in the energy balance sheet, the super heatedsteam can be produced in a heat exchanger by the hot exhaust gases. Thecombustion-supporting air can be isothermally compressed to any requiredpressure, then heated by the hot exhaust gases to any technicallyfeasible extent.

The reciprocating internal combustion engine operating in accordancewith the process according to the invention is characterised by; aseparate cooled compressor providing substantially isothermalcompression of the combustion-supporting air; an exhaust-gas-heated heatexchanger for heating the compressed combustion-supporting air andpossibly for steam-raising; a valve diverting some of the compressedheated air; provision for carburetting and/or compressing a fuel by thediverted air; separate combustible-gas and air passages; valves whichact on both the latter passages and which serve to time the supply ofgas and air to a combustion chamber disposed in the hot cylinder head,the same being thermally insulated from the cylinder, in whichcombustion chamber the gas-air mixture compressed in the top-dead-centerzone is burned by self-ignition and without increase of pressure, andexpanded; and a known exhaust valve for exhausting the expandedcombustion gases from the combustion chamber into the heat exchangerduring the return stroke of the piston.

Advantageously, the compressor for compressing the combustion-supportingair can be a cooled multistage reciprocating compressor whose intakevalve is controlled by a critical high-pressure controller in the highpressure line and which has recoolers between its discrete compressionstages. The reciprocating compressor, which is a separate unit, can beexternally driven, e.g. by an electric motor; advantageously, however,the reciprocating compressor is directly coupled with the engine crankshaft. A single compressor can supply, possibly with the assistance of apressure accumulator, a number of cylinders of an engine withcombustion-supporting air.

The ratio of air to fuel, and therefore maintenance of optimumcombustion conditions, are controlled by an exhaust gas sensor whichdetects the percentages of unburnt hydrocarbons and carbon monoxideand/or carbon dioxide and nitric oxide contents in the exhaust gasescontinuously, and by thermostats in the exhaust pipe and in the cylinderhead. The exhaust gas sensor and the thermostats act jointly on a finalcontrol element which operates the valve appropriately and thus adaptsthe air-fuel ratio to instantaneous operating conditions.

To improve the combustion process, more particularly to lower the flametemperatures near the burner, the heat exchanger comprises an evaporatorwhich is flowed round by the hot exhaust gases and whose high-pressureoutlet is connected either directly to the burner by way of a separateline or to the high-pressure air line.

When liquid fuels, such as gasoline or diesel fuel, are used, thefuel-preparing provision or facility can take the form of a carburettersupplied with the liquid fuel by way of an injection line and a pump. Sothat the engine may be operated on fuels of different consistencies andwhich cannot be gasified without residue, it may be advantageous toprovide two carburetters in parallel with one another which can bebrought into operation alternately, it being possible for one of thecarburetters to be cut out of operation and cleaned or possibly evenreplaced without the engine having to be stopped.

Of great assistance in the endeavours of the invention to improve engineefficiency is a cylinder head formed, in extension of the or each of itscylinders, with recesses into which the top part of the piston extendsunguided and so as to bound a narrow annular gap in the top regions ofits stroke and in which combustion occurs at a substantially constantpressure. Thermally, the technical advantage of such a cylinder headlies in the possiblility of insulating the head from the cooledcylinders of the cylinder block and thus of retaining the heat in theuncooled walls of the cylinder-head recesses surrounding the combustionchambers without shortening the working life of the engine.

The recesses are so devised that when the or each piston is in itstop-dead center position the gap which must be present between therecess end wall and the piston end wall in conventional reciprocatinginternal combustion engines to receive the highly compressed fuelmixture is absent. To reduce heat stressing of the cylinder-headmaterial and in particular of the piston end wall, the recess innerwalls can have a thin coating of a highly refractory material; however,the inner diameter of the coated recesses must remain slightly largerthan the piston diameter so as leave the annular gap.

To further reduce the unwanted removal of heat from the combustionchamber to the cylinder block, the pistons can in conventional manner behollow pistons and can have heat insulation on their inner walls.

Since no work of compression has to be carried out in the cylinder andthe combustible or burned gases are introduced into the cylinder onlyduring each working stroke, every other piston stroke -- i.e., everyoutward stroke -- is a working stroke. This feature leads to the furtheradvantage that an exhaust valve provided e.g. in the cylinder head canremain open throughput the return stroke, thus ensuring uniform removalof the expanded combustion gases during the return stroke of the piston.

Since combustible gas from the gasification facility andcombustion-supporting air are supplied to the nozzle through flowpassages in a cylinder head heated to the self-igniting temperature ofthe mixture and since combustion may be initiated only during or shortlybefore the top-dead center position of the piston, a specially devisedtimed valve must be provided. Such a valve ensures that the two gasesmerge for the first time in the burner nozzle, where they experiencevery intensive eddying. Since combustion occurs in the nozzle withoutany increase of pressure, there is no risk of backfiring in the event ofincomplete gasification of the fuels and of the presence of air in thecombustible-gas line.

The valve providing timed mixing of the two gases can be e.g. a valve ofthe kind used in a similar form in high-pressure steam engines.Considerable advantages, however, are provided by a valve of a kindwherein a tappet guided vertically in a corresponding recess in thecylinder head bears by way of bottom inclined surfaces on inclinedsurfaces of the cylinder-head recess, the latter surfaces being devisedas valve seats. Extending to each of the cylinder-head inclined surfacesis a flow passage for the combustible gas and a flow passage for thecombustion-supporting air. At its bottom end the tappet has a portionwhich divides the top part of the nozzle in two chambers, the side wallsof such portion being shaped to produce eddying. The portion is guidedby way of its curved narrow sides by the burner walls and thus preventspremature mixing of the two gases when the valve is open. Of course, onevalve each can be provided for controlling the supply of combustible gasand the supply of combustion-supporting air, with the consequentadvantage of separate control of the quantity of each substance.

Embodiments of the invention will be described in greater detailhereinafter with reference to the drawings wherein:

FIG. 1 is a vertical section through an embodiment of the reciprocatinginternal combustion engine according to the invention, for use withfuels gasifying without leaving residue;

FIG. 2 is a diagrammatic view of the parallel-connected facilities foruse with fuels which can not be gasified without leaving a residue;

FIG. 3 is a vertical section through another embodiment of the engineaccording to the invention, for gaseous fuels compressed in a separatecompressor;

FIG. 4A is a vertical section through the valve and burner used in theengines shown in FIGS. 1 and 3;

FIG. 4B is a bottom view of the valve shown in FIG. 4A, and

FIG. 5 is a diagrammatic front view of the engine according to theinvention.

The engine shown in FIG. 1 comprises two separate units or groups, viz.,a multistage reciprocating compressor 1 and an internal combustionengine 2 with its appropriate ancillaries. The pistons of both units arein conventional manner connected by connecting rods to a single crankshaft 3.

Walls 4 of compressor 1 are formed with flow passages 5 for a coolant.Interposed between outlet 6 of the low-pressure stage and inlet 7 of thehigh-pressure stage is a recooler 8, in the form of a casing 9 and pipecoils 10. Disposed in high-pressure line 11 is a pressure-limiting valve12 which acts in conventional manner on a valve 13 in intake 14 of thelow-pressure stage.

Combustion-supporting air which has been compressed substantiallyisothermally flows through high-pressure line 11, as indicated by anarrow 15, to a heat exchanger 16; the hot exhaust gases flow through anexhaust pipe 17 in exchanger 16 and are then discharged to atmospherevia a line 18. To ensure very thorough heat exchange, the high-pressureline 11 in the heat exchanger 16 is embodied as a pipe coil. Alsoprovided in the heat exchanger 16 to heat the combustion-supporting airis an evaporator 20 in which water or wet steam is heated and possiblycompressed so as to reduce the flame temperatures in the burner.

Disposed in the exhaust line 18 are an exhaust gas sensor 21 and athermostat 22 whose observed values are transmitted to a final controlelement 23 acting on a three-way valve 24; some of the heated and highlycompressed combustion-supporting air reaching valve 24 is divertedthrough a line 25 to a facility 26 for gasification of liquid fuelsupplied by way of a line 27 and pump 28. The resulting combustible gasleaves through a line 29.

Most of the combustion-supporting air flows from valve 24 through a line31, at least some of which is contrived in cylinder head 30. The gasline 29 and the air line 31 extend to inclined surfaces 32,33respectively of a valve seat (see FIG. 4). Bearing on surfaces 32,33 isa cylindrical tappet 34 which is guided in a recess 35 in the cylinderhead. The bottom inclined surfaces of tappet 34 close or open the twolines 29,31 simultaneously. Tappet 34 terminates in a bottom web orportion or the like 36 which subdivides the top part of a combustionchamber length-wise into a chamber 38 for the combustion-supporting airand a chamber 39 for the combustible gas, with the result that there isintensive mixing of the two gases, and therefore ignition, below itsfree end in the burner nozzle 37.

As indicated in FIGS. 1 and 3, cylinder head 30 is formed with acylindrical recess 40 into which top part 41 of piston 42 extends.Recess 40 is the combustion and expansion chamber. Its diameter isslightly greater than the diameter of the piston top part 41 so that thepiston moves unguided in recess 40, leaving an annular gap R. The pistonis guided by ordinary piston rings 43 which are disposed in the bottompart of piston 42 and which engage with the inner walls of conventionalcylinder liners. To keep down cylinder wall temperatures to the valuesnecessary for the lubricants, the cylinder 45 has air-cooling ribs orfins 46. Of course, the cylinder walls 45 can be cooled by liquidcoolants, e.g. water.

To maintain the required high temperatures in the cylinder-head recess40 and to inhibit any appreciable heat transfer from cylinder head 30 tothe cooled cylinder block, an insert 47 providing considerable heatinsulation is provided at the junction. The inner walls of recess 40 andat least the top end wall of piston 42 are coated with a thin covering50 of a highly refractory material so as to maintain the thermalstressing of the cylinder-head material within limits. The side walls ofthe piston top part 41 can be coated with the refractory material. Toinhibit intensive heat radiation by the cylinder walls into the interiorof the hollow piston 42, such interior has heat insulation 52 at leastnear the top part of the piston.

The exhaust line 17 to the heat exchanger merges into the end wall ofthe cylinder-head recess 30 and is closed and opened by a conventionaltimed exhaust valve 53.

The engine according to the invention operates as follows:

Air supplied through intake line 14 is compressed in the cooledmultistage compressor 1, the heat of compression which is evolved beingremoved substantially by cooling of the piston walls and by the recooler10, so that compression proceeds substantially isothermally. The air,which can be compressed to any technically feasible pressure, is heatedas far as possible in the heat exchanger by the hot gases, then goes tothe three-way valve 24. The quantity of air diverted thereby gasifiesthe liquid fuel supplied through line 27 in the facility 26 and forcesthe highly-compressed hot combustible gas through line 29, when thetappet 34 is at its raised position, into chamber 39 above burner 37(see FIGS. 4A and 4B),

Most of the combustion-supporting air flows from the valve 24 throughthe line 31, when the tappet 34 is in its raised position, to thechamber above the nozzle 37, such chamber being bounded by the tappetportion 36. Because of the sinuous construction of the side walls ofportion 36, when the tappet 34 is open both gases have imparted to thema whirl which leads to intensive eddying in the nozzle 37. Combustionthen occurs as a result of collisions between the hot molecules ofcombustible gas and oxygen, such molecules being at a temperature beyondthe self-ignition temperature and above the temperature of the hot wallsof the nozzle or of the cylinder head and cylinder crown, without anyincrease in pressure, the duration of combustion corresponding, assuminga constant input of combustible gas and air to the nozzle 37, to apredetermined angle of crank shaft rotation -- i.e., to a predeterminedamount of piston travel. If the cylinder-head recess walls which are incontact with the combustion gases have been given an economicallyfeasible protection against heat loss, the surfaces of such walls have atemperature at full engine load of approximately 1000° C. Expansion ofthe combustion gases and the associated temperature decrease gives thegas the possibility and sufficient time to burn the final unburntmixture gas residues at a very low excess of air coefficient since theentire combustion chamber is bounded by hot wall surfaces, the knowndifficulties of complete combustion of the combustible gases which areon or near the cooled walls being prevented by excessive cooling do notoccur.

A highly refractory insulating layer of a thickness of from 3-4/10mm canbe applied to the walls of the cylinder-head recess and of the pistontop part by plasma spraying; such a layer can store the heat energy inthe highest temperature range until most of such energy has beenrestored to the gases as they cool during expansion. The quantity ofheat which does not have to be removed by the engine coolant remainsavailable to the thermo dynamic process and so does not have to be"topped up" by fuels.

If flame temperatures in the burner become excessive when high-energyfuels are used, it may be expedient either to mix thecombustion-supporting air with high-pressure steam produced in theevaporator 20 or to supply such steam through separate lines to theburner or to the fuel-gasifying facility.

FIG. 2 is a diagrammatic view of a facility for gasifying fuel, thefacility comprising two identical units 60,61. One line each 62,63extends from the line 25 to each unit 60,61, the lines comprisingselectively operable shut-off elements 64,65. Discharge lines 66,67having corresponding shut-off elements 68,69 extend to line 29. Fuel issupplied by way of line 27 and pump 28 and a three-way valve 70 to eachof the units 60 or 61 at choice. With a gasification unit thus devised,fuels which do not gasify without leaving residue can be used, sinceeach of the gasification or carburation units can be cut out ofoperation by appropriate operation of the various valves or shut-offelements and cleaned or replaced without operation of the engine havingto stop. Solids fuels, such as coal or the like and thickly viscousunpurified fuels, such as tar, fuel oil, old oil, etc., can be used withfacilities of this kind to drive internal combustion engines. Ifrequired, instead of the two units 60,61 even more units can be providedand connected in parallel. In special cases a connection can be providedbetween the two units 60 and 61 by the valve 70 to ignite the freshmaterial introduced into the adjacent chamber.

The engine shown in FIG. 3 corresponds in its main items to the engineshown in FIG. 1 and so there is no need to repeat the general commentshere. The embodiment shown in FIG. 3 is of use for burning gaseousfuels. The same are compressed in a small multistage reciprocatingcompressor 75 substantially isothermally; the compressor 75 can also bedriven by the crank shaft 3. Accordingly, a recooler 76 is providedbetween the low-pressure stage 77 and high-pressure stage 78. Both thecompressor 1 and the compressor 75 compress their respective medium tothe same pressure, e.g. from 40 to 50 atmospheres gauge. Thecombustion-supporting air and the combustible gas are heated in two pipecoils around which the hot exhaust gases flow; quantities are socontrolled by a controller 81 that the required mixture of gas and airwhich burns at a constant pressure with self-ignition arises in theburner 37. Combustion proceeds without pressure increase until inletvalve 34 closes; piston 42 moves in the cylinder through a distancecorresponding at full load substantially to that volume of a dieselengine occupied by the burnt gases after ignition and combustion of theinjected diesel oil. To increase engine power, the or each inlet valvepermits "filling" only to the extent necessary to obtain such power, asis the case e.g. with reciprocating steam engines. The or eachcompressor provides only the quantities of gas required for immediateoperating circumstances. Closure of tappet 34 is followed by pureexpansion of the combustion gases without appreciable after-burning. Theuse of a separate cooled compressor 75 for the combustible gases and thefact that the same are heated by the exhaust gases in the heat exchanger16 mean that any gas can be burnt safely at high pressure with optimumefficiency.

As in the case of the embodiment shown in FIG. 1, the valve facilityshown in FIG. 4A is responsible for the timed supply and formation ofthe mixture in the nozzle 37.

FIG. 5 shows that the depth of the recess 40 in the cylinder head 30should correspond as far as possible to the piston stroke, thus ensuringthat the combustion chamber exists only in the thermally insulatedcylinder head, so that only a very small amount of heat reaches theengine coolant.

When the engine is cold and the air and gas are unheated, the ignitioncan be provided by a continuous arc of a spark plug or hot plug near thegas and air entries into the cylinder. External ignition is maintaineduntil operating temperature is high enough for self-ignition.

The engine according to the invention can run to either hand ofrotation.

I claim:
 1. An internal combustion process wherein combustion-supportingair is compressed outside of a working cylinder-piston and compressedair and a gaseous fuel are supplied separately in predeterminedquantities to a burner nozzle and mixed therein and ignited and burntwithout increasing the pressure in the cylinder-piston and wherein theenergy of the combustion gases which expand without cooling istransmitted to the working piston and the exhaust gases are removed fromthe working cylinder, characterized in that the combustion-supportingair is compressed substantially isothermally then heated by the hotexhaust gases with some of the compressed hot air being diverted topreheat and carburet and compress the fuel with the remainder of thecombustion-supporting air and the combustible gas evolved being burntsubstantially stoichiometrically by self-ignition.
 2. A processaccording to claim 1, characterized in that the heat evolved in theisothermal compression is removed by a coolant.
 3. A process accordingto claim 1, characterized in that the cylinder walls are cooled onlyduring the bottom part of the piston stroke.
 4. A process according toclaim 1, characterized in that the combustible gas formed from theliquid fuel and the proportion of combustion-supporting air, and thecombustion-supporting air, ignite immediately upon contact with oneanother.
 5. An internal combustion engine in which a combustible gas anda combustion-supporting air are supplied precompressed to acylinder-piston by separate channels and at least one controlled burnernozzle, the mixture formed in the burner nozzle being burnt in thecylinder-piston without substantial pressure increase, characterized bya separate air compressor for providing substantially isothermalcompression of the combustion-supporting air; an exhaust-gas-heated heatexchanger for heating the compressed combustion-supporting air; a valvefor diverting some of the compressed heated combustion-supporting air tothe burner nozzle; means for carburetting and compressing a fuel andreceiving the remainder of the diverted air; means connecting, said fuelcarbureting and compressing means with said burner nozzle; a combustionchamber in the head of said cylinder which is thermally insulated fromsaid cylinder and being in communication with said burner nozzle and inwhich the combustible gas and air which have been compressed separatelyare in the top-dead-center region of said cylinder head burnt byself-ignition; an exhaust valve for exhausting the expanded combustiongases from the combustion chamber into the heat exchanger during thepiston return stroke, the power being controlled steplessly from idlingto full power by variations in filling produced by operation of thevalves.
 6. An engine according to claim 5, characterized in that thevalve controlling the gas/air supply to the combustion chamber has atappet which is guided in a recess in the cylinder head and which whenin its closed position bears by way of bottom inclined surfaces onmatching inclined seating surfaces and which terminates in a portion,which when in the raised position, subdivides the top part of thenozzle-like combustion chamber into a gap for combustion-supporting airand a gap for combustible gas, the line for the combustion-supportingair terminating near the inclined surface and the line for thecombustible gas terminating near the inclined surface of the valve seat.7. An engine according to claim 6, characterized in that those walls ofthe tappet portion which bound the gap chambers are shaped to produce awhirling flow and to cause intimate eddying of the gases in the nozzle.8. An engine according to claim 5, characterized in that the aircompressor is cooled.
 9. An engine according to claim 5, characterizedin that the air compressor is a multi-stage compressor.
 10. An engineaccording to claim 9, characterized in that recoolers in the form ofcooling chambers are provided between the individual compression stagesof the compressor.
 11. An engine according to claim 5, characterized inthat a limit pressure controller controlling the inlet valve is disposedimmediately the downstream from the compressor.
 12. An engine accordingto claim 5, characterized in that the compressor is coupled to the theengines output shaft.
 13. An engine according to claim 9, characterizedin that the multi-stage piston compressor is directly connected to theengine crankshaft.
 14. An engine according to claim 5, characterized inthat the heat exchanger comprises a pipe coil which is flowed around bythe hot exhaust gases and is connected to the compressor delivery by apressure line.
 15. An engine according to claim 1, characterized in thatan element controlled by the waste gas and temperature and which variesthe air-fuel ratio is associated with the diverting valve.
 16. An engineaccording to claim 5, characterized in that the heat exchanger containsan evaporator which is flowed around by the hot exhaust gases and whosevapour outlet is connected to the air line.
 17. An engine according toclaim 5, characterized in that the means for preparing the combustiblegas is a carburettor for liquid fuels which is connected via aninjection line to a compressing pump.
 18. An engine according to claim5, characterized in that two carburettor units adapted to be broughtinto operation alternately are provided in parallel.
 19. An engineaccording to claim 5, characterized in that the cylinder head isfashioned with separate flow passages for the combustion-supporting airand for the combustible gas, such passages merging in a controlledburner nozzle disposed in the cylinder head.
 20. An engine according toclaim 5, characterized in that the cylinder head is formed with at leastone recess in which the top part of the piston extends unguided and inwhich combustion occurs at a substantially constant pressure.
 21. Anengine according to claim 5, characterized in that the cylinder head isthermally insulated from the cooled cylinder walls which guide thepiston bottom part.
 22. An engine according to claim 20, characterizedin that the inner walls of the cylinder-head recess and the outer wallsof the piston top part are coated with a highly refractory material. 23.An engine according to claim 5, characterized in that to form a narrowannular gap the diameter of the piston top part is less then thediameter of the recess in the cylinder head.
 24. An engine according toclaim 5, characterized in that the inner walls of the pistons haveinsulation.
 25. An engine according to claim 5, characterized in thatthe cylinder head has at least one exhaust valve which during eachpiston return stroke is open for at least substantially the entire timetaken for the return stroke.